1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic method or an electrostatic recording method, such as a copying machine or a printer.
2. Description of the Related Art
Conventionally, there are known a number of electrophotographic methods. Generally speaking, the electrophotographic method forms an electrostatic latent image on an image bearing member with a photoconductive substance by using various units. Subsequently, the electrostatic latent image is turned into a toner image by a developing apparatus, and the toner image is transferred to a transfer material such as paper as needed. Then, the toner image is fixed to the transfer material by heat, pressure, etc. to thereby obtain a copy. In addition, the residual toner image on the image bearing member is recovered by a cleaning member.
Regarding the cleaning member, blade cleaning, fur brush cleaning, or roller cleaning are widely used. However, in all of these cleaning methods, the residual toner is either mechanically scraped off or intercepted to be gathered in a waste toner container.
In a method in which mechanical cleaning is thus performed, it can happen that some external additive released from the toner surface due to fluctuations in torque during the rotation and drive of the image bearing member may enter the interior of a contact nip between the cleaning member and the image bearing member. The external additive entering into the interior of the nip adheres to the surface of the image bearing member by the pressure of the cleaning member applied thereto. Once something adheres to the surface of the image bearing member, the adhering substance cannot be scraped off by the cleaning member but may pass while pushing up the cleaning member. If the cleaning member is pushed up, the cleaning performance may declines and an issue (defective cleaning) may occur in which the recovered transfer residual toner is drawn to be allowed to pass.
To cope with this issue, U.S. Pat. No. 6,405,015 discusses the use of a cleaning blade as the cleaning member which exhibits a value of 200 kgf/cm2 or more in a tensile stress test at extension of 300%. Thus, there is discussed a technique capable of suppressing defective cleaning and adhesion of toner or an external additive to the image bearing member by the use of the cleaning blade.
Japanese Patent Application Laid-Open No. 2004-069814 discusses a technique in which a discharge device is provided on the upstream side in the photosensitive-member rotational direction of the cleaning device and in which the surface potential of the image bearing member is varied within a range of 0±200 V, so that cleaning property for the residual toner and silica, which serves as the external additive can be improved.
However, due to the recent increase in the speed and service life of printers, copying machines or the like and the variety in the environments in which they are used, the opportunity for the image bearing member to be worn by the cleaning member tends to increase. The cleaning member is rubbed against the image bearing member, and minute asperity is generated on the surface of the image bearing member, which leads to deterioration in the cleaning performance. In particular, the external additive released from the toner surface is not easily removed by cleaning due to its smaller particle size as compared with that of the toner. Thus, when there exists such asperity on the surface of the image bearing member, the external additive is allowed to pass by the cleaning member to come to exist on the surface of the image bearing member as an adhering substance.
A region having such an adhering substance may cause an issue of defective cleaning, in which the toner is allowed to pass the region, due to poor contact between the cleaning member and the image bearing member. When the defective cleaning occurs, the toner which has been allowed to pass appears on the next image to be formed, thus generating an image defect. In the case where a contact development method is adopted, the toner on the development roller may adhere to the adhering substance at the developing position, so that there is the possibility of the adhering toner being output as an image defect in the form of black dots. Thus, the technique discussed in U.S. Pat. No. 6,405,015 may not be capable of performing complete cleaning on the asperity due to wear of the image bearing member surface while it is effective for a flat image bearing member because of the improvement in the cleaning performance of the cleaning blade.
According to the technique discussed in Japanese Patent Application Laid-Open No. 2004-069814, the surface potential of the image bearing member is varied in a range of 0±200 V to thereby improving the cleaning performance. However, the technique of Japanese Patent Application Laid-Open No. 2004-096814 is directed to an improvement in the cleaning efficiency for silica, and the removal of an adhering substance other than silica through cleaning is not taken into account.